What would be the difference between using a knife and using a dagger in combat?
I know you included your name, but I’ve anonymize it, because I don’t want this to sound like a teardown of you.
“Knife” is a catch all term. It includes an incredibly diverse range of bladed implements, ranging from kitchen utensils to combat tools and technical equipment. So, it wouldn’t be entirely inaccurate to say that there’s no such thing as, “a knife,” in any definitive sense; there are many knives, not just one.
Daggers are, slightly more uniform than that. They usually refer to double edged knives intended for use in combat; and yes, daggers are knives. These might be offensive weapons or parrying tools, and each of those are different weapons. These would later evolve into more specialized variants, like the stiletto; which is a dagger, and not a dagger.
That’s the problem. These aren’t two separate weapons. Not like asking, “what’s the difference between a sword and an axe,” for example.
There is a legitimate question here: “How do you use different kinds of knives in combat?” Unfortunately, the answer is almost as diverse. There are knives designed to parry your opponent’s mainhand weapon. There are narrow blades designed to be inserted through gaps in their armor. There are broad blades, designed to carry themselves via their weight. There are serrated blades designed to do as much tissue damage on the way in and out. This is before you consider curved blades designed to slice more effectively, straight blades for piercing stabs, and a dizzying array of different combinations of designs.